use more caching on travis
use more caching on travis

This allows the latest version of numpy, scipy, and libsbml to be
directly built from pypi, but without extremely long compilation times.
These packages will no longer need to be maintained at
http://opencobra.github.io/pypi_cobrapy_travis/

JuBra/cobrapy,jeicher/cobrapy,jerkos/cobrapy,zakandrewking/cobrapy,JuBra/cobrapy,aebrahim/cobrapy,jerkos/cobrapy,aebrahim/cobrapy,jeicher/cobrapy,zakandrewking/cobrapy